Conventionally, for example, in radio communication devices, such as base station devices or the like, an error sometimes occurs in the frequency of the radio signal to be transmitted due to, for example, aged deterioration of an internal part in the devices. Furthermore, in recent years, studies have been conducted on a system that connects, by optical fibers, radio equipment control (REC) devices that mainly perform a baseband process on signals and radio equipment (RE) devices that mainly perform a radio process on signals by using, for example, an interface called a Common Public Radio Interface (CPRI). In a radio communication system that uses the CPRI, an error sometimes occurs in the frequency of the radio signal sent from an RE device caused by, for example, aged deterioration of optical fibers.
In the radio communication system, for example, because the frequency defined by law is needed to be used, in general, a periodic inspection is performed to determine whether the frequency error described above is within the range that satisfies the definition. However, because the base station devices and the RE devices are often arranged at a high place, such as on the roof of a building, on a steel tower, or the like, the efficiency of measuring the frequencies sent from these devices by maintenance workers on site is low and the cost is increased. Thus, for example, it is conceivable to arrange the reference oscillation source, such as the Rubidium oscillator source or the like, inside a device and use a method of measuring a frequency error by using a measuring circuit that measures a frequency error of a transmission signal by using the reference signal sent from the reference oscillation source.    Patent Document 1: Japanese Laid-open Patent Publication No. 10-31042    Patent Document 2: Japanese Laid-open Patent Publication No. 08-32507
However, when a frequency error is measured by arranging, for example, a measuring circuit, inside a device, the size of the circuit in the device is increased and the cost is also increased. In particular, because the reference oscillation source that generates stable reference signals is comparatively expensive, the implementation cost for accurately measuring a frequency error is increased. In this way, arranging an additional measuring circuit or the like results in an increase in the size of the device and the cost, which is not practical.